Photometry, or photometric measurement, of a radiation source is commonly carried out by spectrometers. A spectrometer uses a slit structure to control the amount of light entering the spectrometer, and uses a diffraction grating in combination with a collimator and a correcting lens to focus spectral components on an imaging plane. A photodetector can be placed on the imaging plane to detect each individual spectral component. However, the results obtained by the photodetector of the spectrometer with this configuration are not useful under certain conditions as a fairly high level of stray light tends to enter into the slit and reach the diffraction grating, thus affecting the result of diffraction grating. Accordingly, the light source needs to be well controlled, yet this limits the wide application of conventional spectrometers.
FIG. 6 is a schematic diagram of a conventional spectrometer 100. As shown in FIG. 6, the conventional spectrometer 100 includes a light source 110, an input unit 120, a collimating lens 130, a planar grating 140, a focusing lens 150, and a linear photodetector 160. An optical signal 200 emitted from the light source 110 enters into the spectrometer 100 through the input unit 120 and travels in free space. After collimation by the collimating lens 130, the optical signal 200 reaches the planar grating 140. The macroscopic profile of a diffraction pattern 142 of the planar grating 140 is a plane. The planar grating 140 is commonly produced by a traditional manufacturing method in which the diffraction pattern is formed by diamond ruling. However, with this manufacturing method, a diffraction grating with a curved surface that is capable of focusing light cannot be produced. Therefore, after the planar grating 140 separates the optical signal into multiple spectral components, a focusing lens 150 is necessary to focus the resulting spectral components on the linear photodetector 160. As a result, the optical path of the spectrometer 100 is rather long. Accordingly, the size of the conventional spectrometer tends to be large. On the other hand, since a large amount of light can enter into the conventional spectrometer, the influence of stray light is relatively small. Thus, it may not be necessary to consider the issue of stray light influence on the signal for detection in conventional spectrometers.